1. Field of the Invention
Embodiments of this invention are generally related to safety valves. More particularly, embodiments of this invention pertain to subsurface safety valves configured to control fluid flow through a production tubing string.
2. Description of the Related Art
Safety Valves are designed to minimize the loss of reservoir resources or production equipment resulting from catastrophic subsurface events by shutting in the well. The “standard” safety valve achieves this by design with one “active control line”. The normally closed safety valves are controlled from the surface via a hydraulic control line that extends from the valve, through the wellhead to a surface controlled emergency closure system. Hydraulic pressure PC applied through the control line maintains the valve in the opened position. Removal of control line pressure returns the valve to its normally closed position. Setting depth directly affects the operational characteristics of the valve due to the hydrostatic pressures PH created from the normal control system.
Conventional safety valve design incorporates a hydraulic piston and spring to open and close the valve. The hydraulic chamber housing the piston is connected to the surface by a hydraulic control line. Pressure is applied to this control line to hold the valve in the open position. Hydrostatic or “head” pressure PH is always present in the control line due to the column of fluid between the safety valve and the surface.
Functionally, control line pressure PC actuates a piston which is mechanically linked to a “flow tube”. The flow tube traverses across a closed flapper thus opening the flow through the safety valve and its tubing. When the surface pressure is released, a return spring returns the valve back to its closed position. The nature of the design is such that the tubing pressure PT, which acts against the active control line piston effect, will assist in valve closure.
To open the valve, hydraulic pressure PC is applied to the upper end of the piston, via the control line, forcing the flow tube downward, opening the flapper.
To close the valve, the applied hydraulic pressure PC is removed from the upper end of the piston. There are two forces available now to force the flow tube upward allowing the flapper to close. The spring now furnishes an upward force FS sufficient to counteract the downward force due to the hydrostatic pressure PH of the fluid in the hydraulic control line. This causes the flow tube to move upward allowing the flapper to close. Tubing pressure PT at the safety valve will also apply an upward force on the hydraulic piston. This will assist the piston in the upward movement of the flow tube allowing the flapper to close.
In a deep set application, the active control line hydrostatic pressure PH is significant, such that a spring may not be able to overcome the hydrostatic pressure, thus not allowing the flapper to close. To compensate for the active control line's hydrostatic pressure PH, a second “balance” line is used to negate the affect of hydrostatic pressure PH from active control line. In existing balance line valves, the second line acts on the underside of the piston, to balance the hydrostatic pressure PH. However, in this design, since the underside of the piston is in fluid communication with the balance line, it is no longer in fluid communication with the tubing; thereby the beneficial effect of the tubing pressure PT is not utilized.
Therefore, there is a need for a safety valve that balances the control line hydrostatic pressure PH while still utilizing the tubing pressure PT to aid in closure of the valve.